This invention relates generally to an electrophotographic printing machine, and more particularly concerns a system for registering two images that are not precisely placed in the same location on the photoreceptor of the printing machine. The registration of the system runs asynchronously over a period of time and resynchronizes in two different pitch modes.
The marking engine of an electronic reprographic printing system is frequently an electrophotographic printing machine. In an electrophotographic printing machine, a photoconductive member is charged to a substantially uniform potential to sensitize the surface thereof. The charged portion of the photoconductive member is thereafter selectively exposed. Exposure of the charged photoconductive member dissipates the charge thereon in the irradiated areas. This records an electrostatic latent image on the photoconductive member corresponding to the informational areas contained within the original document being reproduced. After the electrostatic latent image is recorded on the photoconductive member, the latent image is developed by bringing toner into contact therewith. This forms a toner image on the photoconductive member which is subsequently transferred to a copy sheet. The copy sheet is heated to permanently affix the toner image thereto in image configuration.
Multi-color electrophotographic printing is substantially identical to the foregoing process of black and white printing. However, rather than forming a single latent image on the photoconductive surface, successive latent images corresponding to different colors are recorded thereon. Each single color electrostatic latent image is developed with toner of a color complementary thereto. This process is repeated a plurality of cycles for differently colored images and their respective complementarily colored toner. Each single color toner image is transferred to the copy sheet in superimposed registration with the prior toner image. This creates a multi-layered toner image on the copy sheet. Thereafter, the multi-layered toner image is permanently affixed to the copy sheet creating a color copy. The developer material may be a liquid or a powder material.
In the process of black and white printing, the copy sheet is advanced from an input tray to a position inside the electrophotographic printing machine where a toner image is transferred thereto and then to an output catch tray for subsequent removal therefrom by the machine operator. In the process of multi-color printing, the copy sheet moves from an input tray through a recirculating path internal to the printing machine where a plurality of toner images is transferred thereto and then to an output catch tray for subsequent removal. With regard to multi-color printing, a sheet gripper secured to a transport receives the copy sheet and transports it in a recirculating path enabling the plurality of different color images to be transferred thereto. The sheet gripper grips one edge of the copy sheet and moves the sheet in a recirculating path so that accurate multi-pass color registration is achieved. In this way, magenta, cyan, yellow, and black toner images are transferred to the copy sheet in registration with one another.
Because color printing systems generally require four passes of the copy sheet through the transfer station (once per color) precise matching of the copy sheet with the latent image on the photoreceptor is necessary during each pass of the copy sheet through the transfer station. Current color systems, however, consistently image the photoreceptor in the same position, thus insuring uneven wear of the photoreceptor. In addition, the current color systems may be constrained to run in only one pitch mode, thus reducing throughput.
U.S. Pat. No. 4,578,331 to Ikeda et al discloses an electrophotographic color image forming process wherein three light beams, each representing image information of three primary colors to be recorded by color separation, are simultaneously projected and written to the surface of a photosensitive member. The images are then developed by toners of three different colors and are then printed by transfer printing on a transfer printing sheet.
U.S. Pat. No. 4,935,788 to Fantuzzo et al assigned to Xerox Corporation, discloses a multi-color printing system wherein a plurality of different color images are developed on a photoconductive surface, transferred to an intermediate member in superimposed registration with one another and then transferred to a sheet and fused thereto.
U.S. Pat. No, 4,990,969 to Rapkin discloses a method of forming multi-color images wherein a primary imaging member is used to form a series of primary toner images which are transferred to secondary image members, one for each primary color. The images are then transferred back to the primary imaging member in registration prior to being transferred to a receiving sheet.
U.S. Pat. No. 5,014,094 to Amitani discloses a color image forming apparatus that uses four laser beam printing mechanisms as plural image forming mechanisms, each of which has a photosensitive member. An image is formed on each photosensitive member. A transfer sheet is then moved under the printer mechanism where it receives from the photosensitive members, primary color images sequentially and superimposedly, to form one multi-color image.
While the above-mentioned color printing systems allow for the transfer of a plurality of colors to photoreceptors and copy sheets, there continues to be a need for a system for more precisely matching the latent images on the photoreceptor with the copy sheet, particularly if the latent images are not precisely placed on the photoreceptor.